Hi
While it may appear that functions such as Mid() "cannot", be missing, this is not in fact so. While YOU may not have to define them, since they are built in functions, they are defined somewhere, and that somewhere is an object library.
With Access much of the functionality is held in Object Libraries, if you open any code module in design view, then choose Tools \ References from the menu, you will see a list of available and selected libraries.
What has been suggested to you is that you check this lsit to see if any of the selected (ie ticked) libraries have the word "Missing" alongside them.
In addition I was saying that I do recall seeing a Microsoft Knowledge base article on this subject. If my memory serves me correctly, the problem was applicable to PC's which had more than one version of office installed.
Regards
Ken Reay
Freelance Solutions Developer
Boldon Information Systems Ltd
Website needs upgrading, but for now -
UK